The present invention relates generally to gas turbine engines, and, more specifically, to noise reduction therein.
In an aircraft turbofan gas turbine engine, air is compressed, mixed with fuel, and ignited for generating hot combustion gases which flow downstream through turbine stages and are discharged as a jet with corresponding thrust. The turbines power a compressor, and also power a fan which produces a majority of the propulsion thrust for powering an aircraft in flight.
The propulsion air and jet exhaust are two sources of engine noise. The reduction thereof is an environmental objective and is typically accomplished using various techniques including higher bypass ratio turbofans, acoustic liners inside the engine ducts and fan nacelle, and exhaust mixers.
A principal source of fan noise is the interaction between the fan rotor blades and stator vanes or struts downstream therefrom. The fan has a rotational speed N and effects a blade passing frequency (BPF) which is the product of the rotational speed N and the number B of rotor blades. The fan air generates spinning mode noise produced by rotating pressure fields and wakes caused by the rotor blade, and the interaction thereof with the adjacent stator vanes. This noise is emitted from the engine both upstream through the inlet of the fan duct and downstream through the duct outlet.
The spinning mode noise occurs at discrete frequencies including the fundamental blade passing frequency BPF, which is also a first harmonic, and higher order frequencies or harmonics thereof.
In order to ensure the decay of spinning mode noise, it is known to select the number of vanes to be greater than or equal to twice the product of the number of blades and the blade passing frequency harmonic number. For example, decay of the first harmonic spinning mode noise may be achieved using a number of vanes greater than or equal to twice the number of blades. Higher harmonic noise decay requires even a greater number of vanes which is impractical to implement in a turbofan engine.
Notwithstanding the various techniques available for reducing fan and jet noise in a turbofan engine, residual noise is still significant.
Accordingly, it is desired to further reduce fan and jet noise in an improved manner.